Watch Your Attitude
As the station orbits, it rolls slowly around its long axis at the rate of four degrees per minute (turning a full 360 degrees over a 90-minute orbit). That keeps communications antennas and Earth-facing windows pointing straight down as Earth curves away underneath, and results in a consistent attitude with respect to the ground.
How does the ISS sense its position? Five times a second, onboard rate gyroscopes monitor how fast the vehicle's position is changing. To determine the station's altitude, position, and velocity, data from GPS receivers on the U.S. side are combined with data from Russian navigation satellites, as well as information from sun, star, and horizon sensors on the Russian segment of the station. All these positioning data are fed to onboard computers, which in turn determine how much correction the CMGs need to make.